This invention relates to improved heat-resistant light alloy articles intended for use as internal combustion engine pistons and similar parts, and a method for manufacturing the same.
As is well known in the art, the so-called light alloys such as aluminum alloys and magnesium alloys are characterized by their light weight, but have low heat resistance and poor heat insulation which make it difficult to form such light alloy materials into parts for use in high-temperature environment. To eliminate these shortcomings in order that light alloys may be used in the manufacture of those parts which require heat resistance and insulation as well as light weight, for example, internal combustion engine pistons and combustion chamber-defining cylinder heads, attempts have heretofore been made to provide a light alloy body with a heat resistant and insulating layer on its surface. Such methods are generally classified into the following three types. The first method is by preforming a ceramic material or refractory metal and joining the preform to a piston body of light alloy by mechanical fastening such as bolt fastening and crimping, or by welding. The second method uses insert casting process by which a ceramic material or refractory metal is integrated with a piston body of light alloy. The third method is by coating or treating the surface of a light alloy body by any technique of metallization or spraying, anodization and electrodeposition. However, none of the above-mentioned conventional methods have provided fully successful results. More specifically, light alloy materials such as aluminum and magnesium alloys have an appreciably higher coefficient of thermal expansion than ceramic materials and refractory metals used to form a heat-resistant and heat-insulating surface layer, and this differential thermal expansion causes the surface layer to crack or peel off during thermal cycling, giving rise to a problem in the durability of such articles. Particularly when ceramic materials are used as the heat-resistant and heat-insulating layers in the first and second methods mentioned above, fabricating and processing of ceramic materials are necessary. However, since ceramic materials are generally difficult and expensive to fabricate and process, the overall cost of manufacture is increased. On the other hand, when refractory metals are used as the surface layers, it is difficult to obtain light alloy articles having satisfactory heat insulation because refractory metals themselves are less heat insulative. Moreover, the third method, that is, surface coating or treating method is difficult to form a surface layer having an effective thickness without sacrifice of cost, also failing to achieve satisfactory heat insulation.
Therefore, an object of the present invention which is made in consideration of the above-mentioned circumstances is to provide improved light alloy articles which take advantage of the inherent light weight of light alloys themselves, have excellent heat resistance, heat insulation and durability, and can be produced less costly in high yields. Another object of the present invention is to provide a method for producing such improved light alloy articles.